1. Field of the Invention
The present invention relates to a fiber type wavelength converter and module which converts a fundamental wave to a second harmonic.
2. Description of the Related Art
Hitherto, a laser beam is converted to a second harmonic by using a nonlinear optical material. As a wavelength converter which effects such wavelength conversion, a bulk crystal type one is known. However, since the bulk crystal type wavelength converter utilizes birefringence of the crystal of the nonlinear optical material for phase matching of the fundamental wave and the second harmonic, a nonlinear optical material with no birefringence cannot be used even if it has large optical nonlinearity.
Recently, a nonlinear optical material having large nonlinearity but no or a little birefringence can be used, and a fiber type wavelength converter in which phase matching of the fundamental wave and the second harmonic is easy is used. The fiber type wavelength converter comprises a cladding made of glass and a core which is in the cladding and made of a single crystal nonlinear optical material.
As the nonlinear optical material to be used as the core of the fiber type wavelength converter, one having a large nonlinear optical coefficient is preferably used so as to increase a wavelength conversion efficiency. Examples of such nonlinear optical material are organic nonlinear optical materials such as 2-methyl-4 nitroaniline (MNA), p-nitro-(2-hydroxymethylpyrolinyl)phenylene (NPP), 3,5-dimethyl-1-(4-nitrophenyl)pyrazole (PRA), 4-(N,N-dimethyl)-3-acetaminonitrobenzene (DAN), 4 -nitrobenzylidene-3-acetamino-4-methoxyaniline (MNBA), etc. Japanese Parent Kokai Publication No. 279225/1989 discloses a compound including methyl 4-hydroxybenzoate as nonlinear optical materials.
In optical recording medium such as optical discs, a recording density of the medium depends on a wavelength of a light source. An upper limit of the recording density increases in inversely proportional to a square of the wavelength of the light source as the wavelength becomes shorter. For example, a semiconductor laser in the 0.8 .mu.m band is used as a reproducing light source for the optical disc. If a semiconductor laser in the 0.4 .mu.m band which is in a blue light region and produced from a combination of the nonlinear optical element and the semiconductor laser in the 0.8 .mu.m band is used as the reproducing light source for the optical disc, the recording density of the optical disc can be increased by four times.
When the second harmonic is produced by using the fiber type wavelength converter, in order to increase the conversion efficiency from the fundamental wave to the second harmonic, it is necessary to coincide a polarization direction of the fundamental wave with a direction of the largest nonlinear optical coefficient of the single crystal which is used as the core of the fiber type wavelength converter. To this end, an fiber end face and a cleavage plane of the single crystal should be coincided with each other in a fiber type wavelength converter comprising a singe crystal of a compound which has good molecule orientation in the crystal and in which a direction of the largest nonlinear coefficient is in the cleavage plane such as MNA, NPP and MNBA.
However, the conventional nonlinear optical materials have large absorption in the visible light region including the 0.4 .mu.m band. For example, since MNA, NPP and PRA have large light absorption in wavelength regions around 480 nm, 500 nm and 450 nm, they cannot generate the 0.4 .mu.m band semiconductor laser with high output.
In the conventional fiber type wavelength converter which comprises the core made of the single crystal of the nonlinear optical material and the module comprising such converter, since not only the cleavage plane of the single crystal of the core and the fiber end face are perpendicular to each other but also molecules of the organic nonlinear optical material form hydrogen bonds together with hydroxyl groups present on an inner wall surface of the glass tube cladding, the polarization direction of the fundamental wave does not coincide with the direction of the largest nonlinear optical coefficient, so that the conversion efficiency from the fundamental wave to the second harmonic is low.